As is known for media with shape anisotropy, heating may result in liquid crystalline phases, called mesophases. The individual phases differ by the spatial arrangement of the molecular centers on the one hand, and by the molecular arrangement in respect of the long axes on the other hand (G. W. Gray, P. A. Winsor, Liquid Crystals and Plastic Crystals, Ellis Horwood Limited, Chichester 1974). The nematic liquid crystalline phase is distinguished by only one orientation long-range order existing through parallel arrangement of the long axes of the molecule. Provided that the molecules forming the nematic phase are chiral, the result is a cholesteric phase in which the long axes of the molecules form a helical superstructure perpendicular thereto (H. Baessler, Festkxc3x6rperprobleme XI, 1971). The chiral moiety may either be present in the liquid crystalline molecule itself or be added as doping substance to the nematic phase, inducing the cholesteric phase. This phenomenon was first investigated on cholesterol derivatives (for example H. Baessler, M. M. Labes, J. Chem. Phys., 52, (1970) 631; H. Baessler, T. M. Laronge, M. M. Labes, J. Chem. Phys., 51, (1969) 799; H. Finkelmann, H. Stegemeyer, Z. Naturforschg. 28a, (1973) 799; H. Stegemeyer, K. J. Mainusch, Naturwiss., 58, (1971) 599, H. Finkelmann, H. Stegemeyer, Ber. Bunsenges. Phys. Chem. 78, (1974)) 869.
The cholesteric phase has remarkable optical properties: a high optical rotation and a pronounced circular dichroism which arises due to selective reflection of circularly polarized light within the cholesteric layer. The colors which are apparently different depending on the angle of view depend on the pitch of the helical superstructure, which in turn depends on the twisting ability of the chiral component. In this connection it is possible to alter the pitch, and thus the wavelength range of the selectively reflected light, of a cholesteric layer in particular by changing the concentration of a chiral doping substance. Cholesteric systems of this type provide interesting possibilities for practical application. Thus, it is possible by incorporating chiral moieties into mesogenic acrylic esters and orienting in the cholesteric phase, e.g. after photopolymerization, to prepare a stable, colored network, although the concentration of chiral component therein cannot then be changed (G. Galli, M. Laus, A. Angelon, Makromol. Chemie, 187, (1986) 289). It is possible by admixing noncrosslinkable chiral compounds with nematic acrylic esters and by photopolymerization to prepare a colored polymer which still contains large amounts of soluble components (I. Heyndricks, D. J. Broer, Mol. Cryst. Liq. Cryst. 203, (1991) 113). It is furthermore possible, by random hydrosilylation of mixtures of cholesterol derivatives and acrylate-containing mesogens with defined cyclic siloxanes and subsequent photopolymerization to obtain a cholesteric network in which the chiral component may comprise up to 50% of the material employed; however, these polymers still contain distinct amounts of soluble materials (F. H. Kreuzer, R. Maurer, Ch. Mxc3xcller-Rees, J. Stohrer, Presentation No. 7, 22nd Meeting on Liquid Crystals, Freiburg, 1993).
DE-A 35 35 547 describes a process in which a mixture of cholesterol-containing monoacrylates can be converted by photopolymerization into cholesteric layers. However, the total amount of chiral component in the mixture is about 94%. Although the mechanical stability of such a material, as pure side-chain polymer, is not very great, the stability can be increased only by highly crosslinking diluents.
Besides the nematic and cholesteric networks described above, also known are smectic networks which are prepared in particular by photopolymerization of smectic liquid crystalline materials in the smectic liquid crystalline phase. The materials used for this are, as a rule, symmetrical liquid crystalline bisacrylates as described by, for example, D. J. Broer and R. A. M. Hikmet, Makromol. Chem. 190, (1989) 3201-3215. However, these materials have very high clearing points of  greater than 120xc2x0 C. so that there is a risk of thermal polymerization. Piezoelectric properties can be obtained by admixing chiral materials when an Sc phase is present (R. A. M. Hikmet, Macromolecules 25, 1992, 5759).
The publication by H. Kxc3x6rner and C. K. Ober in Polymer Materials, Science and Engineering, 73 (1995) 456-457 discloses, for example, liquid crystalline cyanates which are thermosetting. Furthermore, Progress in Polymer Science 18 (1993) 899-945, authors E. E. Barclay and C. K. Ober, likewise discloses corresonding liquid crystalline compounds which have epoxides as reactive groups.
Thermally crosslinkable cholesteric liquid crystalline systems have not hitherto been described.
The present invention relates to sheet-like structures obtainable by thermal curing and having a crosslinked cholesteric liquid crystalline ordered structure.
The sheet-like structures according to the invention have a superstructure like that of cholesteric liquid crystals. Either the superstructure is present even before the crosslinking, or it is formed during the crosslinking. It is produced
a) from chiral nematic liquid crystalline compounds,
b) from a nematic and a chiral liquid crystalline compound,
c) from a nematic liquid crystalline and a chiral non-liquid crystalline compound or
d) from a compound which is not nematic but undergoes a transition during the thermal curing into a nematic liquid crystalline structure, and a chiral compound.
Examples of suitable chiral compounds in this connection are the compounds described in German Patent Application P 19520660.6, with those disclosed in claim 5 being emphasized. These are compounds of the general structure
(Zxe2x80x94Y1xe2x80x94Axe2x80x94Y2xe2x80x94Mxe2x80x94Y3)nXxe2x80x83xe2x80x83I
in which the variables have the following meanings:
A spacer,
M mesogenic groups,
Y1, Y2, Y3 chemical bonds or the groups xe2x80x94Oxe2x80x94; xe2x80x94Sxe2x80x94; xe2x80x94COxe2x80x94Oxe2x80x94; xe2x80x94Oxe2x80x94CO-xe2x80x94; xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94; xe2x80x94COxe2x80x94N(R)xe2x80x94 or xe2x80x94N(R)xe2x80x94COxe2x80x94,
R hydrogen or C1-C4xe2x80x94alkyl groups,
X a radical of the formula 
n 2 to 6 and
Z
a) in at least one case a radical having an isocyanate, isothiocyanate, cyanate, thiirane, aziridine, carboxyl, hydroxyl or amino group and
b) the other radicals are hydrogen or unreactive radicals, where the radicals
L are, independently of one another, C1-C4xe2x80x94alkyl or xe2x80x94alkoxy, halogen, xe2x80x94COxe2x80x94OR, xe2x80x94Oxe2x80x94COxe2x80x94R, xe2x80x94COxe2x80x94NHxe2x80x94R or xe2x80x94NHxe2x80x94COxe2x80x94R, and
the radicals Z, Y1, Y2, Y3, A and M, can, because they are present n times in I, be identical or different.
Examples of individual chiral compounds are: 
where R is OH, OCN, ONC or 
The nematic compounds necessary according to a) to c) must be selected from the large number of known thermally crosslinkable structures, it being necessary to take account of the following aspects:
1. The nematic liquid crystalline compounds should have a sufficiently wide phase range.
2. Miscibility with chiral components mentioned in b) and c) must be ensured.
3. Good miscibility with other thermally crosslinkable liquid crystals is desirable to reduce the crystallization temperature and increase the clearing point.
4. The temperature at which the curing is carried out should be as low as possible, a favorable range being from 80 to 200xc2x0 C., preferably 80 to 130xc2x0 C.
The following compounds which substantially meet these criteria may be mentioned by way of example: 
Combination of compounds A and B permits the melting points to be reduced by comparison with use of the individual components. The same applies to components C and D. Components A and B are cured, as is known, using amines which are added in stoichiometric amount. It is advantageous in this case to use structurally similar amines, preferably diamines, such as 
Since the amine component is, as a rule, not a liquid crystal, the overall system must be inherently balanced so that, on curing, a liquid crystalline system is produced or retained.
Details of the composition of such systems may be found in the examples in which, unless noted otherwise, parts and percentages are by weight.
Cyanates and isocyanates require, by contrast with epoxides, no additional components for the curing.
The sheet-like structures according to the invention are suitable, for example, for decorative coatings, for producing security marks, pigmentary particles, polarizers, color filters and IR reflectors.
The starting materials for the sheet-like structures according to the invention are expediently mixed while cooling, and preferably in dissolved form, until homogeneous and subsequently the solvent is removed. In order to preclude any premature polymerization, it may be appropriate to remove the solvent(s) by freeze-drying. The conditions for the mixing and drying depend on the system and must be selected appropriately.
Suitable solvents should be volatile and must have a good dissolving power for the components.
Examples which may be mentioned are ketones, lactones, esters, ethers, hydrocarbons or halohydrocarbons, such as acetone, methyl ethyl ketone, butyrolactone, methyl, ethyl or butyl acetate, diethyl ether, dioxane, tetrahydrofuran, methyl t-butyl ether, toluene, methylene chloride or chloroform.
The ratio of mixing of nematic component (potentially nematic component)/chiral component depends on the planned use of the sheet-like structures according to the invention. The color in particular is determined by the chiral content, because it is determined by the component itself and its helical twisting power. The examples contain corresponding information.
To prepare the mixtures, the individual components are dissolved in a solvent suitable for freeze-drying, in this case preferably dioxane. The monomer concentrations are from 0.005 to 0.01 mol/1. The parts by volume appropriate for the required composition are taken from these stock solutions, mixed, shock-frozen and then freeze-dried.